Yes, I'd say due to the large cam you prob don't have enough engine vac to properly open up the vac operated secondaries. I don't have much experience working with an overly large cam or a big block for that matter, but I'd say switching to a mechanical secondary carb would prob be a good idea. My buddy has a Grand National with a 454 and he runs an 850 Speed Demon with mechanical secondaries and it works good. Most of his buddy's swear by them (mech secondaries). I don't know how large a cam he runs though, but I believe it's pretty big.
